This invention relates in general to centralized testing of telephone cables and more particularly to a system for measuring from a central office the existence and location of faults in two wire telephone lines connected to that office.
In the service and maintenance of telephone system equipment, one important aspect is a rapid determination of the existence and location of problems directly related to the two wire telephone lines themselves. A telephone system central office has a large number, typically 10,000, two-wire telephone lines extending out from it to subscriber equipment. Each telephone line includes two wires, commonly referred to as "tip" and "ring," a large number of these wire pairs being included in a cable, with the cable jacket being connected to ground. At the central office connections to a 50 volt battery provide enabling power on the line. In the United States, the average length of lines extending from central office is 2.5 miles with a maximum length of approximately 10 miles.
One of the problems associated with maintenance service of the telephone lines lies in the relatively stringent requirements for maintaining the subscribers "on line." If a faulty line is detected or reported and the fault cannot be located and serviced in a relatively short period, then the subscriber is usually reconnected to a new pair of wires in order to keep his equipment in service. When this is done the previous pair of wires remain unused and consequently after a period of time cables which may have as many as 1800 pairs of telephone wires within are being only fractionally utilized. Increasing the demand for telephone connections then requires the very expensive project of laying new cables.
As would be expected, a great variety of approaches have been employed in an attempt to resolve these problems. Most such approaches have employed test signals at frequencies in the range of voice frequencies, since these are the operating frequencies of these lines. One significant problem associated with the use of frequencies of this value for long line testing lies in the difficulty of making determinations of susceptance and conductance for the entire length of the line. Such measurements are needed to determine capacitive loading. The resistive portion of the impedance increases with distance along the line, while the reactance portion decreases with distance along the line and, for frequencies in the order of one kilohertz these impedance values cross at approximately three miles. Accordingly, this distance presents a limiting distance for the determination of characteristics of capacitance loading with a test frequency of 1 kilohertz. Additional problems arise from the necessity of protecting any measurement circuit from damage arising either from continuous direct low impedance connection to the battery through a short to a telephone wire connected to the battery, or from catastrophic high voltage, such as lightning striking the line when it is connected to the measurement circuit.
In order to improve the efficiency of these prior art testing systems centralized computers have been employed both to perform algorithms on various combinations of test results in order to determine particular line faults characterized by specific combinations of measurement results, and also as a storage of the data base which stores all of the updated information concerning the lines emanating from a particular central office. This data base information would normally include the length of the lines, the types of terminations and the types of equipment tied onto a line. Since many of these factors change fairly frequently, this data base needs to be continuously updated.
It is therefore a primary object of the present invention to provide a method and apparatus for testing the quality of telephone lines for distance out to more than 10 miles where the measurement can be made rapidly and economically to provide an indication of both the existance and location of problems on the line.